Rather than showing the constant fields associated with the combo choice in every row of the subform, just show them in the header. You seem to be asking how to copy the constant fields and you do not need to do that, nor should you do it. The fields are constants and don't change with each row so they stay in the table which provides the RowSource for the combo.
A simple way to show the fields is to include them in the RowSource of the combo. Then in the AfterUpdate event of the combo, you can copy them to unbound textboxes in the form's header where the combo it.
Me.TestName = Me.cboType.Column(2)
Me.Unit = Me.cboType.Column(3)
Me.NormalRange = Me.cbo.Type.Column(4)
Keep in mind that the RowSource is a zero-based array so the bound field is normally the first field. You can refer to that as
Me.cboType --- normal method
or
Me.cboType.Column(0) --- not normal but valid
The second column is then .Column(1), the third is .Column(2), etc.